Heat exchangers that utilize a single-phase working fluid to transfer heat from a heat source or to a heat sink are known as single-phase heat exchangers. Single-phase heat exchangers are used in a variety of applications ranging from radiators of conventional automobiles to more exotic water-to-ammonia heat exchangers for sustaining life in outer space, e.g., aboard a space shuttle or a space station. Single-phase heat exchangers are also used in other diverse applications, such as removing waste heat from electronic devices, e.g., microprocessors, cooling fusion reactor diverters and producing slush hydrogen.
Compact single-phase heat exchangers are particularly desirable in applications having relatively high heat fluxes. For example, the continually increasing speeds and complexity of microprocessors cause these microprocessors to generate commensurately increasing amounts of heat. Present generation microprocessors typically have heat fluxes in the range of 5 watts/cm2 to 15 watts/cm2. The next several generations of microprocessors are predicted to have much greater heat fluxes, e.g., on the order of 50 watts/cm2 to 200 watts/cm2 or more. One type of compact heat exchanger contemplated for high flux heat transfer applications is what has become known as a normal-flow heat exchanger (NFHX). Specific embodiments of NFHXs have been previously disclosed by the present inventor, e.g., in U.S. Pat. Nos. 5,029,638, 5,145,001, and U.S. patent Publication No. US-2001-0050162-A1, all of which are incorporated by reference in their entirety as if fully disclosed herein. An NFHX is desirable for applications such as microprocessor cooling because it provides: (1) a single phase heat exchanger having a high surface heat flux capability; (2) a compact heat exchanger in which the working fluid experiences a generally small pressure drop as it passes through the heat exchanger; and (3) a small and lightweight heat exchanger having a high thermal transfer effectiveness.
FIG. 1 shows an alternative heat exchanger plate 20 that may be used to form the core of a NFHX as taught in U.S. patent Publication No. US-2001-0050162-A1. Heat exchanger plate 20 is configured such that a separate spacer plate is not required between adjacent heat exchanger plates. Heat exchanger plate 20 has inlet apertures 22 and outlet apertures 24 for respectively defining portions of inlet and outlet manifolds 26, 28 of the core. However, instead of the interconnecting channels being substantially defined by apertures in a spacer plate, interconnecting channels 30 are defined by recessed regions 32 formed in heat exchanger plate 20. Accordingly, webs 34, marginal regions 36 and partitions 38 of heat exchanger plate 20 are each defined by the full thickness of the material from which the heat exchanger plate is made and fins 40 are defined by a partial thickness of the material. Because portions of heat exchanger plate 20 are full thickness and other portions are partial thickness, a separate spacer plate is not required. It will be understood that heat exchanger plate 20, however, can likewise be comprised of two separate plates, namely a first plate that corresponds to the partial thickness portions and a spacer plate that corresponds to the full thickness regions.
In an NFHX 20 incorporating one or more plates 20, fluid enters NFHX 20 and flows through inlet manifold 26. Fluid then flows down through recessed region 32 and over fins 40 toward (in a direction normal to heat transfer surface 21, hence the name) the heat transfer surface 21 that forms the bottom of NFHX 20. Next, fluid flows away from heat transfer surface 21 and enters interconnecting channels 30 and exits NFHX 20 via outlet manifolds 28.
Heat exchanger plate 20 includes an arrangement of inlet and outlet apertures 22, 24 that locates the majority of flow area within the inlet and outlet manifolds distal from heat transfer surface 21. This arrangement avoids placing significantly-sized apertures adjacent heat transfer surface 21 as taught in U.S. Pat. Nos. 5,029,638 and 5,145,001. Such apertures adjacent heat transfer surface 21 reduce the heat transfer efficiency of plate 20 by reducing the size of fins 40 and/or the cross-sectional area between the heat transfer surface and the fins available for conducting heat therebetween.
Prior art heat exchangers such as NFHX 20 were typically designed so that the temperature gradient in the fluid was aligned with the temperature gradient in the fin (i.e., all the heat transfer to the fluid occurs as the fluid moves towards the heat transfer surface). This was believed to be required to maximize thermal performance of the heat exchanger. In order to align the temperature gradients of the fluid and fins, prior art heat exchangers necessitated certain structural limitations.
In order to maximize heat transfer performance of the heat exchanger NFHX 20, it is desired that the width (transverse to the flowing fluid) of fin 40 utilize as much as practical the full width of the heat exchanger. One example of the aforementioned structural limitations is that the flow area within interconnecting channel 30 reduces the width available for heat exchanging fins 40. In addition, because of the prior art design philosophies with respect to temperature gradients, it was not deemed to be necessary or efficient to include fins 40 within interconnecting channels 30 or in any area where the fluid is flowing away from heat transfer surface 21.
Although the design in FIG. 1 did improve upon previous NFHX designs, the relatively narrow interconnecting channels 30 that are necessary to maximize the area of fins 40 causes an increase the overall pressure drop through NFHX 20. The increased pressure drop within interconnecting channels 30 can cause flow maldistribution over fins 40 thereby degrading heat transfer capabilities. In addition, larger pumps or other more expensive means may be required to increase the inlet pressure as a result of the increased pressure drop. The competition between overall fin area and resulting increased pressure drop ultimately limits the overall performance of prior art heat exchangers such as NFHX 20.